The present invention relates generally to television signal transmission systems, and more specifically, to a television signal transmission system employing both analog and digital signal processing techniques for reduced bandwidth requirements.
Broadcast television has assumed a dominant role in our everyday life as a result of an overwhelming investment in home television receivers. When one thinks of television signals, it is therefore natural to think in terms of the television signal broadcast into the home. More efficient encoding of this signal would free valuable spectrum space. However, the television receiver would probably have to be modified or replaced at prohibitive cost in any scheme in which the television signal were appreciably altered. Accordingly, the present invention is concerned with applications involving point-to-point transmission of picture material, long-lines cable distribution, and satellite transmissions rather than specific applications involving broadcast to the home.
Most early efforts in picture coding used analog techniques in an attempt to reduce the analog bandwidth. Today, complex manipulations used to achieve either transmission security or reduced bandwidth are more easily accomplished by first sampling and digitizing the signal and then handling the signal processing in digital form. The processed signal may then be transmitted directly over a ditial channel or converted back to analog form for transmission over an analog channel. Direct digitization of broadcast television signals requires transmission rates of approximately 100 Mb/s for broadcast quality transmissions, representing an increased bandwidth requirement of about 10 times that used for analog transmission of the analog signal. A survey of tradeoffs involved in applications of digital signal processing of television signals is found in an article by Netravali and Limb entitled "Picture Coding: A Review", Vol. 68 No. 3, pp. 366-406, PROCEEDINGS OF THE IEEE, March 1980.
The required bandwidth in television and other image scanning systems depends upon the rate of change of signal intensity along a line of the scanned image. The scanning rate in conventional systems is uniform, and the bandwidth then depends upon the maximum rate of change needed to achieve an acceptable picture quality. In broadcast television, there is a high degree of correlation of the luminance signal from frame to frame. Nevertheless, camera movement and rapid changes of scene can reduce the interframe correlation appreciably. For teleconferencing and videotelephone type scenes where the camera is stationary and the movement of subjects rather limited, only a small fraction of the video samples change appreciably from scene to scene. Consequently, there would be less frame-to-frame correlation in average scenes transmitted in broadcast TV than in videotelephone or videoconference scenes. Measurements have also indicated that typical variations of signal intensity along a single scan tend to vary in bunches, with little variation over one interval followed by a jump in level to the next interval; during a typical interval which usually exceeds 2% of the line duration, the signal intensity remains substantially unchanged.
Television transmission of a single, fixed scene may be achieved using a slow scan rate. In this case, the transmission bandwidth requirement would be small. However, a slow-scan, narrow bandwidth system would be incapable of transmitting a changing scene without serious degradation of picture quality. The time required to transmit video signal information is inversely proportional to the rate of change of the signal intensity. Thus, various inventors have proposed transmission schemes in which slowly varying information would be transmitted at a rapid scanning rate while rapidly varying information would be transmitted at a slow rate.
The concept of variable velocity scanning for signal transmission in described in U.S. Pat. No. 2,307,728 issued to Mertz. In addition, the applicant is also aware of pertinent in one way or another to the persent invention:
(1) No. 2,664,462 issued to Bedford et al PA1 (2) No. 2,965,709 issued to Cherry et al PA1 (3) No. 2,306,435 issued to Graham PA1 (4) No. 3,204,026, No. 3,384,710, and No. 3,459,886 all issued to G. J. Doundoulakis, as either sole or joint inventor.
In the early days of television, variable velocity scanning, i.e., VVS, was a theoretically feasible concept but not one that could be implemented economically in a practical form. Semiconductor devices were still in their infancy. Large scale integration of multiple functions performed by a compact combination of semiconductor devices was almost beyond imagination.
Several early attempts to implement TV systems incorporating VVS produced disappointing results. These schemes were designed on the premise that the rate of change of the signal from a TV camera could be used to control its scanning velocity, thereby reducing the total bandwidth requirements. The bandwidth requirements to transmit the rate of change information, however, were greater than those of the TV camera output signal. Consequently, a greater bandwidth was actually required than would have been required to transmit the TV signal itself using a uniform scanning velocity.
Successful implementation of the VVS concept depends on the use of storage of the TV camera signal. The delayed signal is used together with the rate of change of the camera signal to produce a variable velocity scan. One of the early analog techniques for implementing the VVS concept is described by Doundalakis in his 1965 patent based on a 2 dimensional line to line comparison of stored data and the use of storage tubes. This technique utilizes an unconventional bidirectional scanning method and dispenses with the retrace used in home television receivers. The technique is extended somewhat in his 1969 patent which describes a system using a comparison of corresponding pixels in three frames. Little has been done, however, to extend his achievement since his methods are primarily applicable to black and white transmissions and encounter significant synchronization problems for even this application.
Since around 1970, new forms of coding systems for transmission of picture information have appeared at a rapid rate. These include systems using predictive coding, transform coding, interpolative and extrapolative coding, statistical coding and other methods not neatly classified. Each of these classifications can be broken down further according to whether the coding method is fixed or adaptive. In spite of the large proliferation of coding methods since the 1960's, it is still attractive to consider new system designs based on variations of the VVS concept.
Without high-speed storage, attempts at bandwidth reduction in television systems employing uniform scan have produced poor results. Application of high-speed semiconductor memories, however, makes it possible to employ a uniform scan and to code the signal intensity information into forms in which transmission of non-changing, redundant information is inhibited. The result is a variable rate transmission (VRT) of information which may take on several forms to achieve the same sort of objectives as those of the VVS. For example, one can digitize the intensity information and compare the magnitude of adjacent samples. For essentially constant intensity, it will suffice to transmit a representation of the magnitude at the beginning of a string of equivalued intensities, timing information marking the start of the string, and the length of the string over which the intensity remains essentially constant. Such a coding scheme is called "run-length'encoding".
The present invention uses an alternate form of run length encoding in which slowly varying information is represented by a reduced number of samples. Rapidly varying information, however, is identified by a rate-tag; and the data at almost every sampling point in the rapidly varying block is transmitted over a fixed block length. By adding rate-tag data to label only the rapidly varying information, slowly varying information can be transmitted rapidly while rapidly varying information is transmitted slowly. The concept of VRT can be enhanced further by using interpolation to reconstruct the data represented by a sequence of discrete levels over successive blocks.